In recent years, ultrasound diagnostic apparatuses are being used for early discovery of arteriosclerosis, vascular diseases, and so on. Specifically, ultrasound diagnostic apparatuses are being used to measure intima-media thickness (hereinafter called IMT) which is the combined membrane thickness of the intima and media of blood vessel walls. Here, when the examiner determines the IMT or presence of plaque, the examiner manually specifies (sketches) the shape of the blood vessel adventitia and blood vessel intima in the ultrasound image, and performs the diagnosis based on this sketch. Specifically, the examiner sketches, on the B-mode image, the shape of the periphery of the adventitia as an adventitia contour line, and further sketches the shape of the periphery of the lumen as the lumen contour line. Then, at the end, the examiner performs the diagnosis for the presence of plaque, and so on, based on the shape of the sketch (see Non Patent Literature (NPL) 1.